Fluoroelastomer copolymers based on vinylidene fluoride, for example copolymers with hexafluoropropylene and, optionally, tetrafluoroethylene, are well-known, commercially available materials which possess unusually high chemical, thermal, and oxidative stability. They are readily cured by the use of diamines or by polyhydroxy compounds in combination with phase transfer agents such as quaternary ammonium or phosphonium salts. These crosslinking processes are possible because of the inherent susceptibility of vinylidene fluoride-containing polymers to attack by base. However, this sensitivity to base attack necessarily limits the utility of such fluoroelastomers to applications not requiring high resistance to base. Likewise, peroxide-curable fluoroelastomers based on vinylidene fluoride that also contain a peroxide-reactive curesite monomer, such as are described by Apotheker and Krusic in U.S. Pat. No. 4,035,565, are similarly reactive to basic materials In practice there are many applications, for example, in o-rings and shaft seals of internal combustion engines using aggressive oils and in oil well applications, in which resistance to base is required.
For these uses, special-purpose, base-resistant, peroxide-curable fluoroelastomers have been developed that do not contain vinylidene fluoride at all, (or if present, only in small amounts) and are based instead on tetrafluoroethylene. Such fluoroelastomers require either incorporation of special curesite monomers or treatment prior to peroxide cure to create radical-reactive sites. Thus copolymers of tetrafluoroethylene and propylene are known (U.S. Pat. No. 3,467,635) but require heat treatment (U.S. Pat. No. 4,148,982) for creation of unsaturation to facilitate peroxide cure. Random copolymers of tetrafluoroethylene, ethylene, perfluoroalkyl perfluorovinyl ethers and a curesite-monomer are described in U.S. Pat. No. 4,694,045. U.S. Pat. No. 3,859,259 and Japanese Examined Patent Kokoku 19325/1985 describe copolymers of tetrafluoroethylene and propylene that also contain vinylidene fluoride comonomer in low concentrations (5-30 mole percent), such that reactivity to base is not as severe as in the more highly fluorinated polymers described above.
Tatemoto, et al., in U.S. Pat. No. 4,158,678 describe the preparation of segmented thermoplastic fluoroelastomers, terminated by iodo-groups, that are composed of polymer or copolymer sequences, connected chemically, in the order of (1) a high-melting, crystallizable, fluorine-containing sequence, (2) a fluorine-containing elastomeric copolymer sequence based on vinylidene fluoride and at least one other fluorine containing monomer, and (3) a crystallizable, fluorine-containing sequence, wherein the iodo groups are liberated during emulsion polymerization carried out in the presence of chain transfer agents having the formula RI.sub.n where R is a fluorocarbon or chlorofluorocarbon residue having 1 to 8 carbon atoms, and n is 1 or 2. These materials may be formed into useful molded objects by typical thermoplastic processing procedures, such as extrusion and injection molding, and do not require chemical crosslinking steps, as do thermoset fluoroelastomers. However, because the elastomeric sequence of the segmented thermoplastic elastomer is based on vinylidene fluoride, these polymers suffer the same sensitivity to attack by strong base as do thermoset fluoroelastomers based on vinylidene fluoride.